Method and apparatus for converting a DC voltage to an AC voltage

ABSTRACT

Embodiments of the present invention are directed to an uninterruptible power supply for providing AC power to a load. In embodiments of the present invention, the uninterruptible power supply includes an input to receive AC power from an AC power source, an output that provides AC power, a DC voltage source that provides DC power, the DC voltage source having an energy storage device, an inverter operatively coupled to the DC voltage source to receive DC power and to provide AC power. The inverter includes first and second output nodes to provide AC power to the load, first and second input nodes to receive DC power from the DC voltage source, a resonant element having a first terminal and a second terminal, the second terminal being electrically coupled to the first output node, a first switch electrically coupled between the first terminal of the resonant element and the first input node, wherein during a first time period, the first switch is controlled to allow an electrical current path to connect the resonant element to the capacitive element, an electrical current of the path storing energy in the resonant element and charging the capacitive element to a first voltage level, and during a second time period, the first switch is controlled to block the current path to cause the stored energy in the resonant element to further charge the capacitive element to a second voltage level during the second time period. The uninterruptible power supply further includes a transfer switch constructed and arranged to select one of the AC power source and the DC voltage source as an output power source for the uninterruptible power supply.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation in part of application Ser.No. 09/311,043 titled “Method and Apparatus for Converting a DC Voltageto an AC Voltage,” filed on May 13, 1999, which is incorporated hereinby reference.

[0002] This application is related to an application titled “ExcessiveLoad Capacitor Detection Circuit for UPS,” filed on Mar. 19, 2001, whichis incorporated herein by reference.

FIELD OF THE INVENTION

[0003] Embodiments of the present invention are directed generally to amethod and an apparatus for converting a DC voltage to an AC voltage.More specifically, embodiments of the present invention are directed tomethods and apparatus for converting DC voltages to AC voltages usingresonant bridge inverter circuits in devices such as uninterruptiblepower supplies.

BACKGROUND OF THE INVENTION

[0004] The use of uninterruptible power supplies (UPSs) having batteryback-up systems to provide regulated, uninterrupted power for sensitiveand/or critical loads, such as computer systems, and other dataprocessing systems is well known. FIG. 1 shows a typical prior art UPS10 used to provide regulated uninterrupted power. The UPS 10 includes aninput filter/surge protector 12, a transfer switch 14, a controller 16,a battery 18, a battery charger 19, an inverter 20, and a DC-DCconverter 23. The UPS also includes an input 24 for coupling to an ACpower source and an outlet 26 for coupling to a load.

[0005] The UPS 10 operates as follows. The filter/surge protector 12receives input AC power from the AC power source through the input 24,filters the input AC power and provides filtered AC power to thetransfer switch and the battery charger. The transfer switch 14 receivesthe AC power from the filter/surge protector 12 and also receives ACpower from the inverter 20. The controller 16 determines whether the ACpower available from the filter/surge protector is within predeterminedtolerances, and if so, controls the transfer switch to provide the ACpower from the filter/surge protector to the outlet 26. If the AC powerfrom the rectifier is not within the predetermined tolerances, which mayoccur because of “brown out,” “high line,” or “black out” conditions, ordue to power surges, then the controller controls the transfer switch toprovide the AC power from the inverter 20. The DC-DC converter 23 is anoptional component that converts the output of the battery to a voltagethat is compatible with the inverter. Depending on the particularinverter and battery used the inverter may be operatively coupled to thebattery either directly or through a DC-DC converter.

[0006] The inverter 20 of the prior art UPS 10 receives DC power fromthe DC-DC converter 23, converts the DC voltage to AC voltage, andregulates the AC voltage to predetermined specifications. The inverter20 provides the regulated AC voltage to the transfer switch. Dependingon the capacity of the battery and the power requirements of the load,the UPS 10 can provide power to the load during brief power source“dropouts” or for extended power outages.

[0007] In typical medium power, low cost inverters, such as inverter 20of UPS 10, the waveform of the AC voltage has a rectangular shape ratherthan a sinusoidal shape. A typical prior art inverter circuit 100 isshown in FIG. 2 coupled to a DC voltage source 18 a and coupled to atypical load 126 comprising a load resistor 128 and a load capacitor130. The DC voltage source 18 a may be a battery, or may include abattery 18 coupled to a DC-DC converter 23 and a capacitor 25 as shownin FIG. 2A. Typical loads have a capacitive component due to thepresence of an EMI filter in the load. The inverter circuit 100 includesfour switches S1, S2, S3 and S4. Each of the switches is implementedusing power MOSFET devices which consist of a transistor 106, 112, 118,124 having an intrinsic diode 104, 110, 116, and 122. Each of thetransistors 106, 112, 118 and 124 has a gate, respectively 107, 109, 111and 113. As understood by those skilled in the art, each of the switchesS1-S4 can be controlled using a control signal input to its gate. FIG. 3provides timing waveforms for the switches to generate an output ACvoltage waveform Vout (also shown in FIG. 3) across the capacitor 130and the resistor 128.

[0008] A major drawback of the prior art inverter circuit 100 is thatfor loads having a capacitive component, a significant amount of poweris dissipated as the load capacitance is charged and discharged duringeach half-cycle of the AC waveform. This power is absorbed by theswitches S1, S2, S3, S4, which typically requires the switches to bemounted to relatively large heat sinks. The issue of power dissipationbecomes greater for high voltage systems, in which the energy requiredto charge the load capacitance is greater. The dissipation of power inthe switches dramatically reduces the efficiency of the inverter, andaccordingly, reduces the run-time of the battery 18 in the UPS 10. Therise in temperature of the switches also becomes a large concern.

SUMMARY OF THE INVENTION

[0009] In one general aspect, the present invention features anuninterruptible power supply for providing AC power to a load having acapacitive element. The uninterruptible power supply includes an inputto receive AC power from an AC power source, an output that provides ACpower, a DC voltage source that provides DC power, the DC voltage sourcehaving an energy storage device, an inverter operatively coupled to theDC voltage source to receive DC power and to provide AC power, theinverter including first and second output nodes to provide AC power tothe load having the capacitive element, first and second input nodes toreceive DC power from the DC voltage source, a resonant element having afirst terminal and a second terminal, the second terminal beingelectrically coupled to the first output node, a first switchelectrically coupled between the first terminal of the resonant elementand the first input node, wherein during a first time period, the firstswitch is controlled to allow an electrical current path to connect theresonant element to the capacitive element, an electrical current of thepath storing energy in the resonant element and charging the capacitiveelement to a first voltage level, and during a second time period, thefirst switch is controlled to block the current path to cause the storedenergy in the resonant element to further charge the capacitive elementto a second voltage level during the second time period, a set ofswitches operatively coupled between the first and second output nodesand the first and second input nodes and controlled to generate AC powerfrom the DC power, and a transfer switch constructed and arranged toselect one of the AC power source and the DC voltage source as an outputpower source for the uninterruptible power supply.

[0010] Other features may include one or more of the following: thefirst voltage level is a portion of a voltage source and the secondvoltage level is substantially the voltage source; the set of switchesincludes a second switch electrically coupled between the second outputnode and the second input node, a third switch electrically coupledbetween the second output node and the first input node, a fourth switchelectrically coupled between the first output node and the first inputnode, and a fifth switch electrically coupled between the first outputnode and the second input node; the inverter further includes a sixthswitch electrically coupled between the first terminal of the resonantelement and the second input node; the resonant element includes aninductor; each of the switches includes a transistor; the energy storagedevice includes a battery; and the transfer switch is constructed andarranged to receive the AC power from the input and to receive the ACpower from the inverter and to provide one of the AC power from theinput and the AC power from the inverter to the load.

[0011] In another general aspect, the uninterruptible power supplyincludes an input to receive AC power from an AC power source, an outputthat provides AC power, a voltage source that provides DC power, thevoltage source having an energy storage device, an inverter operativelycoupled to the voltage source to receive DC power and having an outputto provide AC power, the inverter including means for charging thecapacitive element to a first voltage level by creating an electricalcurrent path from the inverter to the load through a resonant element,wherein the resonant element stores energy from an electrical current ofthe path, means for blocking the electrical current path after thecapacitive element has been charged to the first voltage level to causeenergy from the resonant element to be transferred to the capacitiveelement to further charge the capacitive element to a second voltagelevel, and a transfer switch constructed and arranged to select one ofthe AC power source and the voltage source as an output power source forthe uninterruptible power supply.

[0012] Other features may include one or more of the following: theenergy storage device includes a battery; the resonant element includesan inductor; and the transfer switch is constructed and arranged toreceive the AC power from the input and to receive the AC power from theoutput of the inverter and to provide one of the AC power from the inputand the AC power from the output of the inverter to the load.

[0013] In another general aspect, the present invention features amethod of supplying an uninterruptible AC voltage to a load having acapacitive element using an uninterruptible power supply having a DCvoltage source with an energy storage device. The method comprisingsteps of charging the capacitive element to a first voltage level bysupplying electrical current from the DC voltage source to the loadthrough a resonant element in the uninterruptible power supply, storingenergy in the resonant element from the electrical current, blocking theelectrical current from the DC voltage source to the load through theresonant element after the capacitive element has been charged to thefirst voltage level, and transferring the stored energy from theresonant element to the capacitive element to further charge thecapacitive element to a second voltage level.

[0014] Other features may include one or more of: supplying load currentfrom the DC voltage source to the load after the capacitive element hasbeen charged to the second voltage level, blocking the load current fromthe DC voltage to the load after a predetermined period, discharging thecapacitive element through the resonant element, and transferring energyfrom the resonant element to the energy storage device in the DC voltagesource; receiving an AC voltage from an AC power source, selecting oneof the AC power source and the DC voltage source as an output powersource for the uninterruptible power supply; and wherein the resonantelement includes an inductor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a better understanding of the present invention, reference ismade to the drawings which are incorporated herein by reference and inwhich:

[0016]FIG. 1 is a block diagram of a typical uninterruptible powersupply;

[0017]FIG. 2 shows a schematic diagram of a typical prior art invertercircuit;

[0018]FIG. 2A shows a block diagram of a voltage source used with theinverter circuit of FIG. 2.

[0019]FIG. 3 shows timing waveforms for the inverter circuit shown inFIG. 2;

[0020]FIG. 4 shows a schematic diagram of an inverter circuit inaccordance with one embodiment of the present invention;

[0021]FIG. 5 shows timing waveforms for the inverter circuit shown inFIG. 4;

[0022]FIG. 6 illustrates a current path through the inverter of FIG. 4during a charging mode of the inverter corresponding to a starting pointof the positive half cycle of the output voltage waveform;

[0023]FIG. 7 illustrates a current path through the inverter of FIG. 4during a positive half cycle of the output voltage waveform;

[0024]FIG. 8 illustrates a current path through the inverter of FIG. 4during a discharging mode of the inverter at the end of the positivehalf cycle of the output voltage waveform;

[0025]FIG. 9 illustrates a current path through the inverter during anenergy recovery mode of the inverter; and

[0026]FIG. 10 illustrates alternative timing waveforms for the invertercircuit in FIG. 4.

DETAILED DESCRIPTION

[0027] One embodiment of an inverter 200 in accordance with the presentinvention will now be described with reference to FIG. 4 which shows aschematic diagram of the inverter 200 coupled to the voltage source 18 aand the load 126. The inverter 200 includes MOSFET switches S1, S2, S3and S4 of the prior art inverter 100 and includes two additional MOSFETswitches S5 and S6 and an inductor 140. In one embodiment, the switchesS5 and S6 are similar to switches S1-S4 and include a transistor 134,138 having an intrinsic diode 132, 136. Each of the transistors 134 and138 has a gate 115 and 117 that is used to control the state of thetransistor.

[0028] In one embodiment that provides an output of 120 VAC, 400 VA, 25amps peak current to the load from an input to the inverter ofapproximately 170 VDC, the switches S1-S6 are implemented using part no.IRF640 available from International Rectifier of E1 Segundo, Calif. For220 VAC applications, the switches may be implemented using part no.IRF730 also available from International Rectifier. The inductor 140, inthe 120 VAC embodiment, may be implemented using a 1.8 mH inductorhaving a very high Bsat value to be able to withstand high peak currentswithout saturating. In one embodiment, the inductor may be made from anEI lamination structure of M-19, 18.5 mil steel having a large air gapbetween the E and I laminations. Other values of inductors may be usedwith embodiments of the present invention depending upon the peak switchcurrent and physical size of the inductor desired. In selecting aninductor for use, the transition time, or time required to charge ordischarge the load capacitance, should also be considered to prevent thetransition time from becoming either too short or too long. If thetransition time is too long, then the pulse width of the output waveformmay become too long. If the transition time is too short, the peakswitch currents become greater.

[0029] The operation of the inverter 200 to provide AC power to the loadwill now be described with reference to FIGS. 5-9. FIG. 5 provides atiming diagram of the operation of the switches S1-S6 of the inverter200 and also provides the output voltage waveform across the load 126.In the timing diagram of FIG. 5, for each of the switches S1-S6, whenthe corresponding waveform is in the high state, the switch is turned on(conducting state) and when the corresponding waveform is in the lowstate the switch is turned off (non-conducting state).

[0030] In the inverter 200, the switches are shown as being implementedusing NMOS devices. As known by those skilled in the art, for an NMOSdevice, a control signal having a high state is supplied to the gate ofthe device to turn the device on (conducting), while a control signalhaving a low state is supplied to the gate to turn the device off(non-conducting). Accordingly, the timing diagram of each of theswitches also represents the state of the control signal provided to thegate of the corresponding transistor. In embodiments of the presentinvention, the control signals may be provided from, for example,controller 16 of the UPS of FIG. 1 when the inverter is used in a UPS.Alternatively, the control signals may be supplied using timing logiccircuits residing within the inverter itself as is known in the art.

[0031] During a first time period from t0 to t1 in FIG. 5, switches S4and S5 are turned on and switches S1, S2, S3 and S6 are turned offcreating a current path through the inverter 200 in the direction ofarrows 150 as shown in FIG. 6. Only the components of the inverter 200in the current path created during the first time period are shown inFIG. 6. As shown in FIG. 6, with switches S4 and S5 turned on, theinductor 140 and the load 126 are connected in series across the voltagesource 18 a. During the first period, the output voltage across the loadVout rises in a resonant manner from zero volts to the voltage of thevoltage source 18 a. The output voltage Vout is prevented from risingbeyond the voltage of the voltage source by the diode 104 (FIG. 7) ofswitch S1. The diode 104 will conduct current to limit the outputvoltage Vout to the voltage of the voltage source.

[0032] Once the output voltage Vout reaches the voltage of the voltagesource (or shortly thereafter), at time t1, switch S1 is turned on andswitch S5 is turned off. Switches S1 and S4 remain on for a secondperiod from time t1 to time t2, during which time, the load is coupledacross the voltage source 18 a. FIG. 7 shows the current path throughthe inverter during the second time period. As shown in Fig, 7, loadcurrent during the second period follows arrows 154. Also during thesecond time period, the energy that was stored in the inductor duringthe first time period causes the voltage across the inductor to reverseand energy in the inductor is released to a storage device in thevoltage source, such as a battery or a capacitor, through a current thatfollows a path along arrow 156 through diode 104 of switch 1 and diode136 of switch 6. In addition, depending upon the load impedance, currentfrom the energy stored in the inductor may also follow a path throughthe load.

[0033] During a third time period from time t2 to time t3, the voltageacross the load is returned to zero. At time t2, switches S1 and S4 areturned off to disconnect the load from the voltage source and switch S6is turned on to place the inductor effectively across the load as shownin FIG. 8. During the third time period, energy stored in the loadcapacitor 130 is transferred to the inductor 140, and the voltage acrossthe load decreases to zero. The output voltage Vout is prevented fromgoing negative by diode 110 (FIG. 9) of switch S2. The diode 110 willconduct current to limit the output voltage to zero.

[0034] At time t3 switch S6 is turned off, and all switches remain offduring a fourth time period from t3 until t4. The current path throughthe inverter 200 during the fourth time period follows arrows 160 shownin FIG. 9. During the fourth time period, the energy in the inductor 140freewheels into the voltage source 18 a through diodes 110 and 132 of S2and S5, and the voltage across the load typically remains at zero. Thetime from t3 until t4 is normally chosen to be long enough to permit allof the inductor energy to be transferred to the voltage source 18 a.

[0035] During a fifth time period from t4 to t5, switches S1 and S3 areturned on to maintain a low impedance across the load to prevent anyexternal energy from charging the output to a non-zero voltage. This isreferred to as the “clamp” period. At time t5, all switches are againturned off and remain off for a sixth time period until time t6.

[0036] Beginning at time t6, and continuing until time t9 the negativehalf cycle of the AC waveform is created. The negative half cycle iscreated in substantially the same manner as the positive half cycledescribed above in connection with FIGS. 5-9, except that switch S3 issubstituted for switch S4, switch S6 is substituted for S5 and switch S2is substituted for S1. The positive and negative half cycles thencontinue to be generated in an alternating manner to create an AC outputvoltage waveform.

[0037] In the embodiments described above, and in particular withreference to FIGS. 5-7, switch S5 is left on until the load voltage Voutreaches the voltage of the voltage source 18 a. At time t1, switch S5 isturned off and as shown in FIG. 7, the energy stored in the inductor 140freewheels into the voltage source 18 a. However, some of the inductor'sstored energy also freewheels into the load and bus capacitanceresulting in some power loss. In another embodiment of the invention,which will now be described, an alternative timing sequence minimizethis power loss. Furthermore, another benefit of the alternative timingsequence is that lower peak and rms current flows through the resonantcircuit. Thus, the inductor stores less energy and therefore a lowerBsat value may be used along with a smaller inductor. The alternativetiming sequence will now be described with reference to FIG. 10.

[0038] With reference to FIG. 10, during a first time period from t′0 tot′1, switches S4 and S5 are turned on and switches S1, S2, S3 and S6 areturned off creating a current path through the inverter 200 in thedirection of arrows 150 similar to that shown in FIG. 6. With switchesS4 and S5 turned on, the inductor 140 and the load 126 are connected inseries across the voltage source 18 a. During the first time period, theload voltage Vout rises in a resonant manner from zero volts to aportion of the voltage of the voltage source 18 a, preferably,approximately half of the voltage of the voltage source 18 a. At timet′1, switch S5 turns off blocking the current path from the voltagesource 18 a to the inductor 140. During the second time period from t′1to t′2, the inductor 140 freewheels through reverse diode 136 and theenergy stored in the inductor continues to charge the capacitor andincrease the load voltage Vout to the voltage of the source voltage 18a. Accordingly, the power loss due to the inductor's stored energy beingfreewheeled into the bus capacitance is minimized. According to oneembodiment, the controller 16 controls appropriate switches such thatfreewheeling or “swing” time is made approximately equal to the inductorcharge time. For example, if the inductor charge time is 100 us theinductor freewheeling time is set at about 100 us. The output voltageVout is prevented from rising beyond the voltage of the voltage sourceby the diode 104 (FIG. 7) of switch S1.

[0039] Once the load voltage Vout reaches the voltage of the sourcevoltage (or shortly thereafter), at time t′2, switch S1 turns on andswitches S1 and S4 remain on for a third time period from t′2 to t′3,during which time, the load is coupled across the source voltage 18 asimilar to that shown in FIG. 7. At time t′3, switch S1 turns off todisconnect the load from the voltage source 18 a and switch S6 turns onto place the inductor effectively across the load similar to that shownin FIG. 8. During a fourth time period from t′3 to t′4, some of theenergy stored in the load capacitor 130 is transferred to the inductor140 and the voltage across the load decreases to approximately half thevoltage source 18 a, at which time t′4, the switch S6 is turned off.During the fifth time period from t′4 to t′5, with the switch S6 turnedoff, the inductor 140 freewheels through reverse diode 132 and itsstored energy is returned to the voltage source 18 a in a manner similarto that shown in FIG. 9 and finishes discharging the load capacitor tozero volts. The output voltage Vout is prevented from going negative bydiode 110 (FIG. 9) of switch S2. The diode 110 will conduct current tolimit the output voltage to zero.

[0040] During a sixth time period from t′5 to t′6, switch S2 turns onand switches S2 and S4 maintain a low impedance across the load toprevent any external energy from charging the output to a non-zerovoltage. This is referred to as the “clamp” period. At time t′6, allswitches are turned off.

[0041] Beginning at time t′6 and continuing until time t′12, thenegative half cycle of the AC waveform is created. The negative halfcycle is created in substantially the same manner as the positive halfcycle described above in connection with FIGS. to , except that switchS3 is substituted for switch S4, switch S6 is substituted for S5 andswitch S2 is substituted for S1. The positive and negative half cyclesthen continue to be generated in an alternating manner to create an ACoutput voltage waveform.

[0042] In one embodiment of the present invention, in an inverterdesigned to generate 50 Hz voltage waveforms, the first time period fromt′0 to t′1 is approximately 100 microseconds, the second time periodfrom t′1 to t′2 is approximately 100 milliseconds, the third time periodfrom t′2 to t′3 is approximately 4.8 milliseconds, the fourth timeperiod from t′3 to t′4 is approximately 100 microseconds, the timeperiod from time t′4 to t′5 is approximately 100 microseconds, and thetime period from t′5 to t′6 is approximately 4.8 milliseconds. In thisembodiment, the negative half cycle of the waveform is symmetric withthe positive half cycle, and accordingly, the rise time, fall time andduration of the negative half cycle are approximately equal to those ofthe positive half cycle.

[0043] In embodiments described above, during the clamp period from t′5to t′6 after a positive half cycle switches S2 and S4 are turned on toclamp the output to a low impedance. During the clamp period from t′11to t′12 after a negative half cycle, switches S1 and S3 are turned on toclamp the output to a low impedance. In another embodiment of thepresent invention, following a positive half cycle, switches S1 and S3are turned on to clamp and after a negative half cycle, switches S2 andS4 are turned on to clamp. This method is less desirable becausecirculating currents will flow through inductor 140 during the clampperiods, resulting in additional power losses. In a third embodimentduring both clamp periods, switches S1 and S3 are turned on to clamp. Ina fourth embodiment during both clamp periods, switches S2 and S4 areturned on to clamp.

[0044] In embodiments of the present invention, the inverter 200, isused in the manner described above, to create an output AC voltagehaving the waveform shown in FIG. 10 from an input DC voltage using aresonance circuit. The use of the resonance circuit allows the loadcapacitance to be charged and discharged with only a minimum power loss.The only power losses incurred in the inverter 200 are due tocharacteristics of inverter components including the ESR of the inductorand due to series resistance of each of the switches when in the onstate. Thus, inverters in accordance with embodiments of the presentinvention, do not require bulky heat sinks like inverters of the priorart, and are more efficient than inverters of the prior art. Theimproved efficiency of inverters in accordance with embodiments of thepresent invention make them particularly desirable for use inuninterruptible power supplies, wherein they can extend the operatingtime of a UPS in battery mode, reduce the size and weight of the UPS andreduce electromagnetic emissions from the UPS.

[0045] In embodiments of the present invention described above,inverters are described as being used with uninterruptible powersupplies, for example, in place of the inverter 20 in the UPS 10 ofFIG. 1. As understood by those skilled in the art, inverters of thepresent invention may also be used with other types of uninterruptiblepower supplies. For example, the inverters may be used with UPSs inwhich an input AC voltage is converted to a DC voltage and one of theconverted DC voltage and a DC voltage provided from a battery-powered DCvoltage source is provided to an input of the inverter to create the ACoutput voltage of the UPS. In addition, as understood by those skilledin the art, inverters in accordance with embodiments of the presentinvention may also be used in systems and devices other thanuninterruptible power supplies.

[0046] In the inverter 200 described above, MOSFET devices are used asthe switches S1-S6. As understood by those skilled in the art, a numberof other electrical or mechanical switches, such as IGBT's with integralrectifiers, or bipolar transistors having a diode across the C-Ejunction, may be used to provide the functionality of the switches.Further, in embodiments of the present invention, each of the switchesS1-S6 need not be implemented using the same type of switch.

[0047] In embodiments of the invention discussed above, an inductor isused as a resonant element in inverter circuits. As understood by oneskilled in the art, other devices having a complex impedance may be usedin place of the inductor, however, it is desirable that any such devicebe primarily inductive in nature.

[0048] In the embodiments of the present invention described above,energy is returned from the inductor to the voltage source after theload capacitance has been discharged. As understood by those skilled inthe art, the voltage source may include a battery that receives theenergy from the inductor, or the voltage source may include a storagedevice other than a battery, such as a capacitor, coupled in parallelacross the voltage source that receives the energy.

[0049] Having thus described at least one illustrative embodiment of theinvention, various alterations, modifications and improvements willreadily occur to those skilled in the art. Such alterations,modifications and improvements are intended to be within the scope andspirit of the invention. Accordingly, the foregoing description is byway of example only and is not intended as limiting. The invention'slimit is defined only in the following claims and the equivalentsthereto.

What is claimed is:
 1. An uninterruptible power supply for providing ACpower to a load having a capacitive element, the uninterruptible powersupply comprising: an input to receive AC power from an AC power source;an output that provides AC power; a DC voltage source that provides DCpower, the DC voltage source having an energy storage device; aninverter operatively coupled to the DC voltage source to receive DCpower and to provide AC power, the inverter including: first and secondoutput nodes to provide AC power to the load having the capacitiveelement; first and second input nodes to receive DC power from the DCvoltage source; a resonant element having a first terminal and a secondterminal, the second terminal being electrically coupled to the firstoutput node; a first switch electrically coupled between the firstterminal of the resonant element and the first input node, whereinduring a first time period, the first switch is selected to enable anelectrical current path from the resonant element to the capacitiveelement, an electrical current of the path storing energy in theresonant element and charging the capacitive element to a first voltagelevel, and during a second time period, the first switch is selected toblock the electrical current path to cause the stored energy in theresonant element to further charge the capacitive element to a secondvoltage level during the second time period; a set of switchesoperatively coupled between the first and second output nodes and thefirst and second input nodes and controlled to generate AC power fromthe DC power; and a transfer switch constructed and arranged to selectone of the AC power source and the DC voltage source as an output powersource for the uninterruptible power supply.
 2. The uninterruptiblepower supply of claim 1 , wherein the first voltage level is a portionof a voltage source and the second voltage level is substantially thevoltage source.
 3. The uninterruptible power supply of claim 1 , whereinthe set of switches includes: a second switch electrically coupledbetween the second output node and the second input node; a third switchelectrically coupled between the second output node and the first inputnode; a fourth switch electrically coupled between the first output nodeand the first input node; and a fifth switch electrically coupledbetween the first output node and the second input node.
 4. Theuninterruptible power supply of claim 3 , wherein the inverter furtherincludes: a sixth switch electrically coupled between the first terminalof the resonant element and the second input node.
 5. Theuninterruptible power supply of claim 4 , wherein the resonant elementincludes an inductor.
 6. The uninterruptible power supply of claim 5 ,wherein each of the switches includes a transistor.
 7. Theuninterruptible power supply of claim 6 , wherein the energy storagedevice includes a battery.
 8. The uninterruptible power supply of claim7 , wherein the transfer switch is constructed and arranged to receivethe AC power from the input and to receive the AC power from theinverter and to provide one of the AC power from the input and the ACpower from the inverter to the load.
 9. The uninterruptible power supplyof claim 1 , wherein the resonant element includes an inductor.
 10. Theuninterruptible power supply of claim 1 , wherein each of the switchesincludes a transistor.
 11. The uninterruptible power supply of claim 1 ,wherein the energy storage device includes a battery.
 12. Theuninterruptible power supply of claim 1 , wherein the transfer switch isconstructed and arranged to receive the AC power from the input and toreceive the AC power from the inverter and to provide one of the ACpower from the input and the AC power from the output of the inverter tothe load.
 13. An uninterruptible power supply for providing AC power toa load having a capacitive element, the uninterruptible power supplycomprising: an input to receive AC power from an AC power source; anoutput that provides AC power; a voltage source that provides DC power,the voltage source having an energy storage device; an inverteroperatively coupled to the voltage source to receive DC power and havingan output to provide AC power, the inverter including: means forcharging the capacitive element to a first voltage level by creating anelectrical current path from the inverter to the load through a resonantelement, wherein the resonant element stores energy from an electricalcurrent of the path; means for blocking the electrical current pathafter the capacitive element has been charged to the first voltage levelto cause energy from the resonant element to be transferred to thecapacitive element to further charge the capacitive element to a secondvoltage level; and a transfer switch constructed and arranged to selectone of the AC power source and the voltage source as an output powersource for the uninterruptible power supply.
 14. The uninterruptiblepower supply of claim 13 , wherein the energy storage device includes abattery.
 15. The uninterruptible power supply of claim 14 , wherein theresonant element includes an inductor.
 16. The uninterruptible powersupply of claim 15 , wherein the transfer switch is constructed andarranged to receive the AC power from the input and to receive the ACpower from the output of the inverter and to provide one of the AC powerfrom the input and the AC power from the output of the inverter to theload.
 17. The uninterruptible power supply of claim 13 , wherein theresonant element includes an inductor.
 18. The uninterruptible powersupply of claim 13 , wherein the transfer switch is constructed andarranged to receive the AC power from the input and to receive the ACpower from the output of the inverter and to provide one of the AC powerfrom the input and the AC power from the output of the inverter to theload.
 19. A method of supplying an uninterruptible AC voltage to a loadhaving a capacitive element using an uninterruptible power supply havinga DC voltage source with an energy storage device, the method comprisingsteps of: charging the capacitive element to a first voltage level bysupplying electrical current from the DC voltage source to the loadthrough a resonant element in the uninterruptible power supply, storingenergy in the resonant element from the electrical current; blocking theelectrical current from the DC voltage source to the load through theresonant element after the capacitive element has been charged to thefirst voltage level; and transferring the stored energy from theresonant element to the capacitive element to further charge thecapacitive element to a second voltage level.
 20. The method of claim 19, further comprising steps of: supplying load current from the DCvoltage source to the load after the capacitive element has been chargedto the second voltage level; blocking the load current from the DCvoltage to the load after a predetermined period; discharging thecapacitive element through the resonant element; and transferring energyfrom the resonant element to the energy storage device in the DC voltagesource.
 21. The method of claim 19 , further comprising steps of:receiving an AC voltage from an AC power source; selecting one of the ACpower source and the DC voltage source as an output power source for theuninterruptible power supply.
 22. The method of claim 19 , wherein theresonant element includes an inductor.